1. Field of the Invention
The present invention relates to a semiconductor laser device used for optical information processing apparatuses such as an optical disk and a laser printer.
2. Description of the Related Art
The density of the data recorded onto an optical disk becomes higher as the spot size of a light beam focused onto the optical disk becomes smaller. Since the spot size of the light beam is proportional to a light wavelength raised to the power of 2, an oscillation wavelength of a semiconductor laser device serving as a light source should be shortened in order to increase the density of data recorded onto the optical disk. Therefore, great interest has occurred in the reduction in the wavelength of light emitted from a semiconductor laser device. At present, a GaAlAs semiconductor laser device emitting light having a wavelength of 780 nm (infrared) is utilized in a compact disc (CD). An InGaAlP semiconductor laser device emitting light having a wavelength of a 650 nm (red) is utilized in a digital versatile disc (DVD) on which data is recorded at a higher density than in a CD. To further increase a density of data recorded onto a DVD so as to record a high-quality image, a semiconductor laser device emitting light in the blue region is required. Gallium nitride group compound semiconductors have received much attention as semiconductor materials to realize such a semiconductor laser device.
With reference to FIG. 18, a conventional gallium nitride group compound semiconductor light-emitting element (disclosed in Japanese Laid-Open Patent Publication No. 7-162038) will be described. This light emitting element will be fabricated as follows. First, trimethyl gallium (TMG) and NH.sub.3 are supplied at about 500.degree. C. by a metalorganic vapor phase epitaxy (MOVPE) so as to deposit a GaN layer 161 onto a sapphire substrate 160. Next, after raising the substrate temperature to about 1000.degree. C., trimethyl aluminum (TMA) and SiH.sub.4 (monosilane) are additionally supplied so as to deposit an n-type AlGaN cladding layer 162 onto the GaN layer 161. Next, after lowering a substrate temperature to about 700.degree. C., trimethyl indium (TMI), TMG and NH.sub.3 are supplied so as to deposit an InGaN active layer 163 onto the n-type AlGaN cladding layer 162. Thereafter, after raising a substrate temperature to about 1000.degree. C. again, TMA, TMG, cyclopentadienyl magnesium (Cp.sub.2 Mg), and NH.sub.3 are supplied so as to deposit a p-type AlGaN cladding layer 164 onto the InGaN active layer 163.
Next, part of the n-type GaN layer 161, the n-type AlGaN cladding layer 162, the InGaN active layer 163 and the p-type AlGaN cladding layer 164 is selectively dry etched until part of the n-type GaN layer 161 is exposed. As the final step, an n-side electrode 165 is formed on the exposed surface of the n-type GaN layer 161, while a p-side electrode 166 is formed on the p-type AlGaN cladding layer 164.
The light-emitting element described above is a light-emitting diode (LED). Since the light-emitting diode does not have a light confinement structure, the light-emitting diode cannot induce laser oscillation by itself. In order to induce laser oscillation, a resonator should be provided. Normally, a crystal plane, which is formed by cleavage or etching and then processed into a flat mirror, is used as the resonator. Since the refractive index of a gallium nitride semiconductor is about 2.8, a reflectance of the mirror formed by using a gallium nitride semiconductor crystal plane is low, i.e., about 22%. Accordingly, a threshold current of laser oscillation becomes disadvantageously high.